As a next-generation display, electronic paper having an advantage of paper as a display medium has been extensively researched and developed in addition to an organic EL display and an inorganic EL display. Electronic paper is expected to be a rewritable display device that maintains the convenience of paper from the viewpoint of a reduction in weight and flexibility as compared with a panel display. Electronic paper can be driven at a low voltage, and can hold an image displayed by applying a voltage for a long time without consuming power. Therefore, electronic paper can achieve excellent energy efficiency.
As an image display method for electronic paper, a method that utilizes an electrochemical reaction as a fundamental principle, a method that causes pigment particles to undergo electrophoresis, a method that causes pigment particles to undergo magnetophoresis, and the like have been proposed. In particular, an image display method that causes pigment particles to undergo electrophoresis is widely studied as a promising image display method.
For example, a monochrome image is displayed as follows using a microcapsule electrophoresis method. Specifically, a microcapsule is charged with a transparent insulating liquid such as a silicone oil as a migration medium. Negatively-charged titanium dioxide particles (white particles) and positively-charged carbon black (black particles) are dispersed in the silicone oil. A voltage is applied between a transparent top electrode and a bottom electrode so that the charged particles move in opposite potential directions, whereby a monochrome image is displayed.
FIG. 1 is a schematic view showing the image display principle when applying a positive potential to a transparent top electrode and applying a negative potential to a bottom electrode. In FIG. 1, negatively-charged white titanium dioxide particles move toward the transparent top electrode set at a positive potential, and positively-charged black carbon black particles move toward the bottom electrode set at a negative potential. Therefore, the negatively-charged white titanium dioxide particles are observed through the transparent top electrode.
As shown in FIG. 2, when the polarities of the potentials applied in FIG. 1 are reversed, the positively-charged black carbon black particles move toward the transparent top electrode, and the negatively-charged white titanium dioxide particles move toward the bottom electrode, whereby the black carbon black particles are observed through the transparent top electrode. A monochrome image can be displayed by combining the above-described phenomena.
In order to improve image display accuracy, it is important to disperse the carbon black (black particles) in the migration medium with excellent dispersibility. An improvement in dispersibility of carbon black in an aqueous dispersion has been extensively studied mainly aimed at inkjet printers and the like.
On the other hand, when using the microcapsule electrophoresis method, since a correct image is not displayed if the migration medium enclosed in the microcapsule is affected by a potential, it is necessary to use a non-polar migration medium or a migration medium with low polarity. Therefore, a silicone oil is suitably used for the microcapsule electrophoresis method. This makes it necessary to study the dispersibility of carbon black from a point of view differing from that of an aqueous dispersion (polar medium).
JP-A-2004-526210 discloses an electrophoresis medium in which pigment particles are suspended in a suspension fluid, wherein a polymer is chemically bonded to or crosslinked on the pigment particles in an amount of 1 to 15 wt % based on the pigment particles.
The polymer has a main chain and a plurality of side chains bonded to the main chain, and each of the side chains contains at least about four carbon atoms.